1. Field of the Invention
The present invention relates to a transmission control apparatus, for example, a transmission control apparatus for use in a line terminating unit included in an optical access network.
2. Description of the Background Art
Optical access networks have been constituted at low cost by widely using an Ethernet passive optical network (EPON) system. Gigabit Ethernet PON (GE-PON) systems have been widely popularized in Japan as one of the EPON systems. In these days, when network services of GE-PONs are applied in, for example, collective housing and overcrowded areas, GE-PONs capable of multiple branching are necessary in order to reduce the operational cost.
In conventional GE-PONs, upstream bands for use in transmission from optical network units (ONUs) in the premises of subscribers to an optical line terminal (OLT) in a station, or central office, of a telecommunications carrier may be shared by the ONUs currently involved in communications.
As a bandwidth allocation scheme, dynamic bandwidth allocation (DBA) is used. In the DBA, an OLT grasps a traffic volume raised in each ONU and allocates a communication band corresponding to the traffic volume. This attains economical, efficient bandwidth allocation.
With reference to FIG. 2, notification of a traffic volume from each of the ONUs 92-1 to 92-3 to the OLT 91 uses a Report frame, and notification of an allocation band from the OLT 91 to each of the ONUs 92-1 to 92-3 uses a Gate frame. The Report and Gate frames are, for example, standardized by IEEE (Institute of Electrical and Electronics Engineers) 802.3av/ah. Std, as shown in FIGS. 3A and 3B, respectively.
According to FIG. 2, the ONUs 92-1 and 92-2 notify upstream traffic volumes raised themselves to the OLT 91 on Report frames R. The OLT 91 provides time slots for receiving Report frames to receive Report frames associated with the connected ONUs. Then, the OLT 91 determines bands to be allocated to the ONUs 92-1 and 92-2. Bands are allocated in every predetermined period of time called a grant period. Bands thus determined will be allocation bands for use in the grant period successive thereto. Information on the determined bands is delivered to the ONUs 92-1 and 92-2 on Gate frames G. The ONUs 92-1 and 92-2 read out from the Gate frames transmission time for transmitting a Report frame and upstream data of themselves in the successive grant period, and starts transmitting the appropriate Report frame and the upstream data at the transmission time thus read out.
The upstream transmission proceeds in this way. In FIG. 2, RTT represents a period of time from when the OLT 91 starts transmission of Gate frames toward the ONUs to when the OLT receives first a Report frame in reply from an ONU, that is, Round Trip Time. The technique collecting information on a data storage volume from Report frames transmitted by the ONUs 92-1 and 92-2 to conduct DBA calculation is called status reporting DBA (SR-DBA).
The conventional DBA scheme requires the OLT to collect Report frames for all ONUs in the system. That further requires the OLT to transmit Gate frames to all ONUs.
The grant period consists of a Gate and a Data transmission time interval or subperiod. In Data transmission intervals, the OLT transmits Gate frames separated by a predetermined temporal gap from each other, called inter-packet gap (IPG).
According to the Gate frame format shown in FIG. 3A, the field Grant #1 defines a prompt of transmitting a Report frame by means of the start time of the transmission, “Start Time”, and the field Grant #2 defines a prompt of transmitting upstream data bymeans of the length of period of the transmission, “Length”.
The “Start Times” are selected such as to time division-multiplex transmission frames of the ONUs without colliding with each other. The value of “Length” represents the size of Report frame when transmitting a Report frame, and a time duration corresponding to an allocation band when transmitting upstream data. The size of Report frame is fixed as defined by the Report frame format shown in FIG. 3B.
A prompt, or instruction, of transmitting Report frames is directed to all ONUs connected to the OLT. A prompt, or instruction, of transmitting upstream data is directed only to an ONU or ONUs in which upstream traffic is raised.
When a smaller number of ONUs are connected, a small number of Gate frames are transmitted accordingly, so that grant periods are available for transmitting downstream data in the portions thereof proportionate. On an optical network system branching into a lot of optical access networks including a corresponding number of connected ONUs, however, a proportionately increased number of Gate frames may be transmitted. This requires the Grant periods to be occupied by longer Gate transmission times so as to squeeze downstream data transmission times, thereby deteriorating the transmission efficiency of downstream data.
The above problem could be solved by increasing the length of grant periods. However, since the length of grant periods is proportional to delay time in data transmission, this solution would result in an increase in the delay time. Therefore, a transmission control system is required which is improved in efficiency of transmitting downstream data without modifying grant periods.